One of the most costly aspects of IC technology is the labor required to bond the IC chip to the rest of the circuit. Originally this was done by wire-bonding, which required the manual soldering of a wire under a microscope, first to the microscopically small chip and then to the rest of the circuit, one bond at a time. This is not a feasible long-term solution, as the number of bonds necessary would soon far exceed the number of laborers available.
One solution to this problem was to use a gang-bonding technique, wherein automatic bonding of a film-carrying frame to the IC chip or component avoids the necessity of wire bonding. As described in the May 16, 1974 issue of Electronics, page 89, such a technique uses a roll of film that carries to a bonding station a plurality of frames each of which has many generally planar microscopic, spider-shaped copper fingers known as leads. At the bonding station the inner portions of the leads are aligned and then simultaneously connected to the bonding sites on the IC chip. The outer portion of the leads is then available for bonding to remaining portions of the IC package.
Conventionally, such leads are themselves premanufactured, preferably by a photoresist etch process. Generally, such etching has been applied to a three-layer blank, comprising a layer of electrically conductive metal such as copper and a layer of photoresist on each side of the metal. At first both resists were positive-working, usually comprising a quinone diazide. The result of such a blank is a lead frame that lacks any means for maintaining the alignment of the etched leads. That is, the blank provided no means for supporting the etched leads because both resists of the blank were used for etching the leads.
Others suggested that a supporting plastic of the 3-layer blank should be formed to support the etched fingers or leads prior to bonding, as described for example in U.S. Pat. Nos. 3,795,043, issued Mar. 5, 1974 and 3,763,404, issued Oct. 2, 1973. The one layer of the blank that becomes the supporting plastic can be a negative-working resist. A typical negative-working composition has been "Riston", a photoresist material available under the noted trademark from DuPont.
One difficulty with such "supported plastic" blanks has been that, under certain exposure conditions, a large amount, e.g., from 10 to 50 weight percent, of the monomer of the negative resist can be left unpolymerized in the exposed areas. This has been found to be undesirable, because such monomer can leach out and have a corrosive influence on the finished product. Even if corrosive leaching is somehow avoided, residual monomer tends to outgas during the two separate steps of bonding the inner lead portions and the outer lead portions. Such outgassing is thought to be detrimental, particularly to the more expensive, high reliability IC components to be fabricated using the lead frames.
Therefore, it is desirable from the standpoint of electronic reliability that the residual monomers be eliminated. On the other hand, such elimination in effect removes a major plasticizer from the negative resist. This results in poor adhesion and brittleness that can completely negate the negative resist's function of supporting the leads prior to, during and after bonding of the IC chip. Thus, what has been needed is a negative resist formulation which, after substantially all residual monomers are removed from exposed areas, still retains sufficient adhesion to the metal and flexibility as will permit the blank to be coiled and uncoiled during the various process steps.
A large number of photoresist compositions are known in the art for generalized use. Many of these compositions are alleged to provide flexibility or improvement in some other respect, e.g., adhesion. Thus, British patent specification No. 1,507,704 discloses a photoresist composition including certain photopolymerizable monomers and a binder. However, there is no recognition by this patent of resist uses peculiar to the formation of beam leads for IC chips.